Hereinbelow, the air taken at the compression stage of a turbine engine will be able to be called “bleed”. In modern airplanes this hot air can be used to activate de-icing cells, pressurize and heat the cabin, pressurize the hydraulic tanks or pneumatic actuators or even pre-heat the brakes.
In the airplanes, the “bleed” can reach very high temperatures. One problem to be resolved is how to detect the leaks of hot air along ducts in which this air circulates.
In one known solution, detection loops are installed that are made up of heat-sensitive cables having temperature-dependent characteristics. These heat-sensitive cables are installed along ducts in order to be able to react to the changes of temperature induced by leaks. Thus, when a leak occurs in a duct, the flow of hot air impacting on the heat-sensitive cable makes it react.
The detection loop is made up of coaxial cables whose two conductors are insulated by a eutectic salt that is highly insulating in the nominal state but gauged to melt at a specific temperature. This chemical property is reversible. In the case of a leak, the heat-sensitive cable therefore behaves locally as a quasi-short-circuit 2. The closed loop provokes an alert which is sent to the cockpit.
The “leak” information item is transmitted to the maintenance teams. However, this information item does not accurately indicate the location of the leak.
More often than not, a resistance measurement or a capacitance measurement is performed from each end of the loop as illustrated in FIG. 1. By knowing the resistance per unit of length of the cable 1, the point of the cable where the leak has occurred is deduced therefrom from measurements 11 and 12 of resistances R1, R2 performed from each end of the loop. The measurements give:R1=2ρLhot R2=2ρ(L−Lhot)
L being the total length of the coaxial cable, and Lhot being the length from the first end to the hot air leak. The factor 2 takes account of the fact that the lengths Lhot or (L−Lhot) are travelled in outward and return directions by the measurement current to the short circuit.
The length Lhot=L/(1+R1/R2) is deduced naturally therefrom.
In practice, the aging of the cable produces measurement uncertainties. In particular, the cable does not age or degrade uniformly. In effect, the spot increases in resistance per unit of length can occur at certain points of the cable. False alarms also arise whose origin is not clearly identified.
Thus, the solutions of the prior art therefore present a number of drawbacks, in particular:
the locating accuracy is poor;
the nominal resistance may be subject to variation depending on the age and the state of disrepair of the loop;
a continuity measurement requiring access to both ends is required to permanently check that the loop is not cut;
a degradation may arise locally at the junctions of the heat-sensitive cables, increasing the contact resistance and skewing the leak location measurement.